The present invention relates to novel glass compositions which possess various improved properties, including complete absorbance of harmful ultraviolet rays.
Most of the glasses manufactured today are based on three main constituents: silica, alkali metal oxide and oxides of metals belonging to Group II of the Periodic Table (calcium, magnesium, zinc). Silica in its fused state, is an excellent glass but, as the melting point of crystalline silica (i.e. sand) is above 1700.degree. C. and forms glass above 2200.degree. C., its production is very expensive so that its uses are restricted to a specific limited purposes. In order to reduce the melting point of silica, it is necessary to add a flux, such as sodium carbonate which provides the sodium oxide constituent. Thus, by adding about 25% of sodium oxide to silica, the melting point is reduced from 1723.degree. C. to 850.degree. C. and the glass forming temperature to 1400.degree. C. However, such glasses are easily soluble in water. The addition of a third constituent such as calcium oxide, magnesium oxide or zinc oxide, renders the glass insoluble, but too much of this third constituent renders the glass prone to devitrification, i.e. the precipitation of crystalline phases in certain ranges of temperature.
The optimum composition for glass which was considered to be useful for many purposes included 75% silica, 15% alkali oxide and 10% oxides of a metal from Group II. Of course, in addition to these main constituents, other materials are also incorporated in order to impart a specific property; for example, by adding small amounts of cobalt oxide together with traces of arsenic trioxide and sodium nitrate, the green color imparted by the iron impurity, generally present in sand, is substantially eliminated. Glasses of very different compositions are suggested when special physical and chemical properties are required. Among the properties which are particularly important for glass, the following can be mentioned: electrical properties including conductivity and dielectric constant, optical properties and ultraviolet ray transmission.
The electrical conductivity of glass varies with the composition and the temperature used in its manufacture. In most glasses, the current is carried by alkali metal ions moving through the material, but semiconducting glasses have recently been discovered in which the current is carried by electrons.
New uses for glass arise continuously, as also do new developments in the glasses themselves. In 1965, a glass was developed for use in the laser, possessing the property of light amplification by stimulated emission of radiation. In the laser device, it is necessary to have certain ions in surroundings that will permit them to be excited by incident light; the ions will be excited by incident light and will emit radiation of longer wavelength through the process known as fluorescence. When certain critical conditions associated with the electronic processes of the ions are accomplished, it is possible to produce in this way very intense and highly homogeneous beams of light. A glass containing about 5% of neodymium has been found to be suitable for some of these applications.
Scintillating glasses and fibers have been developed in the last twenty years as new materials for electromagnetic calorimetry as well as for tracking applications in high energy physics. Thus, a new glass composition, which is based on a cerium-doped lithium-aluminum-magnesium-silicate, was described by Atkinson et al (Nuclear Instr. Methods in Phys. Res. A 254, 500-514, 1987). It is mentioned that such glasses have a maximum absorption at 320 nm, an energy conversion coefficient of 0.55% which is equivalent to 2.1 photons/Kev.sub.1, refractive index of 1.46 and a fast decay time of about 100 nanoseconds. The main drawbacks of these scintillating glass fibres are: long decay time, low light yield in the range of 0.2 to 0.5, which is much below of that obtained today with various plastic materials and short radiation length of about 9.3 cm compared to 42 cm obtained with some plastic materials.
In another paper (U. Buchner et al, Nuclear Inst. and Methods in Phys. Res. A272, 695-706, 1988), a new scintillating glass electromagnetic calorimeter was described. The composition contained as main components, 44.2% BaO, 42% silica and 1.6% Ce.sub.2 O.sub.3 as a scintillating component. The decay constant for this glass is mentioned to be 87+5 nanoseconds.
According to a German patent No. 3,920,447, a scintillation element for a fibre-optic ionizing radiation detector consists of 5 to 1000 cm long fibres of Ce.sup.+3 -doped quartz glass containing at least 95% SiO.sub.2. It is claimed that this quartz glass has a decay time of about 80 to 90 nanoseconds.
In a review by C. Angelini et.al. (Nuclear Instruments and Methods in Physics Research A281,50-4,1989) the decay time of light emission from cerium-doped scintillation glass is discussed. The requirement of high energy physics experiments for precise tracking detectors combined with the advances of optoelectronic technology have stimulated research for development and use of scintillating fibre detectors with a decay time of much below the usual encountered of 50 nanoseconds.
Another important use of glass which has started to be developed in the last fifteen years, is for solar greenhouses shielding ultraviolet light having a wavelength below 340 m.mu., but transparent for useful UV of 370-390 m.mu.. It was reported that light below this wavelength imparted propagation of various molds causing plant diseases. The standard method of preventing transmission was to attach to the glass using an adhesive, a light-transmitting plastic film, such as polyethylene or polypropylene, which is UV-stabilized, to the inward facing of a glass panel. It is claimed that such panels have improved thermal insulation and thus are saving energy required for heating the greenhouse as well as an improved control of condensation on the panels.
According to the U.K. patent No. 2,093,899, two sheets of glass are suggested for glazing a greenhouse. The characteristic feature for obtaining the desired results is that one of the sheets has a bend which defines an air space between the sheets.
A report on materials for luminescent greenhouse solar collectors is given by Levitt et al (C.A. 88, 155711y). Collectors were made from Neodium-doped laser glass and Rhodamine (a doped plastic). As pointed out, although the results obtained are encouraging, they indicate a need for further spectral sensitization and for reduced matrix-loss coefficient.
In U.S. Pat. No. 5,039,631, glass compositions are disclosed possessing a chemically strengthened surface layer being useful as optical glass that absorbs electromagnetic radiation in the ultra-violet, visible and/or infrared regions of the electro-magnetic spectrum. A characteristic feature of these compositions is the presence of a high content of lanthanide oxide. The strengthenability of the glasses produced is explained by the presence of zinc oxide and sodium oxide and the absence of potassium oxide. The presence of the lanthanide in amounts above 7% is mentioned to be a disadvantage due to the occurrence of unacceptable devitrification. Another constituent mentioned in this patent to be required as a fluxing agent is boron oxide in amount of between 5% to 17% (on a molar basis). Below 5% the meltability property is poor while above 17% phase separation and devitrification occur.
The above brief review clearly indicates the long felt need for novel glass compositions possessing improved properties for the demand of the high-technologies which have been developed in the last few years.